Conventionally, as a frame structure for a vehicle body of a vehicle such as an automobile, there has been well known one in which the plate thickness of the frame is increased or a reinforcing plate material (so-called reinforcement) is disposed within a frame cross section with the aim of enhancing the safety at collisions so that the frame is enhanced in strength and rigidity while being improved in impact energy absorbancy.
Meanwhile, in recent years, particularly from the viewpoint of improving fuel performance, there has been a demand for suppressing increases in the weight of the vehicle body. However, the above conventional frame structure would result in a considerable increase in weight, making it difficult to achieve both the maintening of fuel performance and improvement in collision safety at the same time.
For this reason, discussions have been made to improve the strength and rigidity of the frame and the energy absorbancy at collisions by filling urethane foam as a filler material within at least part of the frame cross section, without incurring any large increase in weight as in the case in which the plate thickness of the frame is increased or a reinforcement is provided. This has been partly put into practice use.
As an example, Japanese Patent Laid-Open Publication HEI 3-32990 discloses a structure in which for a simple and reliable accomplishment of a process of filling urethane foam into the cross section of the frame (center pillar), air vent holes are provided at connecting portions of the center pillar with roof side rails.
When a filler material is filled into the frame cross section for enhancement in the energy absorbancy of the frame at collisions as described above, it has conventionally been practiced to use a filler material having a high deformability above a certain level against action of collision loads, such as urethane foam, so as to implement energy absorption by deformation of the frame including such a filler material.
However, as a result of keen studies and investigations on the improvement in the energy absorbancy of the frame by the application of filler materials, the present inventor has found it difficult to attain a sufficient improvement in the energy absorbancy of the frame when the filler material is low in strength and too deformation-prone like urethane foam. The reason of this is that when a material having a high deformability like urethane foam is used as the filler material as it has been conventionally, collision loads are hardly be sufficiently dispersed and transferred from a load input point to its surrounding frame steel plate, and the frame is largely deformed locally only at the load input point and its vicinities.
As shown above, in attempting to attain an improvement in energy absorbancy by filling a filler material into the frame cross section, its effects would largely vary depending on the type and mechanical characteristics of the filler material.
However, conventionally, a variety of proposals have been made on the way how the filler material given by urethane foam is filled into the frame cross section, or the structure by which the filler material is held, but there have been no proposals so far which discuss material characteristics, especially mechanical characteristics, that the filler material to be used and filled into the frame cross section should have, and which refer to the appropriateness of use of materials to the frame based on the discussions on the characteristics. Further, use of materials other than urethane foam for the filler material has not been concretely discussed, as is the case at present.
It is therefore a primary object of the present invention to allow the energy absorbancy of the frame to be more effectively enhanced, by discussing material characteristics with which a filler material to be filled into the frame cross section should be endowed, and by using a filler material complying with this purpose, in an attempt to improve the energy absorbancy of a vehicle-body frame by means of filler material.